The present invention relates to a power amplifier, especially a radio frequency (RF) amplifier used in mobile handset and other battery-powered applications (e.g., portable wireless devices, personal digital assistants (PDAs), notebook computers, and others). More particularly, the present invention relates to a multiple power mode power amplifier with high efficiency appropriate for amplifying power corresponding to various output power levels without using bypass switching circuits.
Mobile handsets or mobile telephones (also sometimes called cell phones) for wireless communication services are becoming smaller and lighter. This also means a size of the battery, which is a typically a sizeable portion of a mobile handset, is also becoming smaller in order to facilitate a smaller and lighter mobile handset. But at the same time a telephone and its battery is becoming smaller, it is desirable to extend a talk time of a mobile handset. As can be appreciated, these are difficult goals to reconcile; if the power drain of a device remains the same and the battery is smaller, operating time of the device per battery charge becomes shorter.
In a conventional mobile handset, the radio frequency (RF) power amplifier consumes most of the power of the overall system of the mobile handset. Thus, low efficiency of the RF power amplifier degrades the efficiency of the overall system, drains the battery more rapidly, and thus reduces the talk time. For this reason, much research in this field concentrates on increasing the efficiency of the RF power amplifier. If the RF power amplifier is more efficient, this reduces power drain and the battery, which in turn increases talk time or the operating time of the device per batter charge.
A multiple power mode power amplifier is one of the devices introduced recently as a result of such research conducted to increase efficiency of the RF power amplifier. The multiple power mode power amplifier is configured to operate its own power stage corresponding to a desired situation and is operated in one of several operation modes corresponding to output power levels, as discussed in U.S. Pat. Nos. 5,152,004; 5,175,871; 5,276,912; 5,530,923; 5,661,434; 5,758,269; 5,909,643; 6,060,949; 6,069,526; and 6,356,150, all of which are incorporated by reference. Bypass switching circuits have been used for such operations of the multiple power mode power amplifier.
If low output power is required, it is desirable to adjust a path of power transmission to bypass a power stage. In contrast, if the high output power is required, it is desirable to adjust a path of power transmission to pass the power stage in order to provide high output power. Using a conventional multiple power mode power amplifier (which uses bypass switches) that selectively performs mode transition corresponding to desired output power levels, it is possible to reduce DC power consumption at the time of transferring signals of low output power.
However, the multiple power mode power amplifier with switches (e.g., PIN diodes) is relatively costly to manufacture and the switches reduce efficiency somewhat because they may have negative gain (e.g., 1dB gain per switch). Further, more than one power stage among a plurality of power stages connected to each other in serial should be switched in order to implement the multiple power mode power amplifiers, and more than one bypass switching circuits and a complex logical control circuit for controlling the bypass switching circuits are required for the switching operation.
Power losses caused by switching operations at the bypass switching circuits causes a reduction of output power and the reduction of output power causes reduction of efficiency of the multiple power mode power amplifier. Further, there is another problem in that an adjacent channel power ratio (ACPR) gets worse. Furthermore, the size of the entire system gets larger due to bypass switching circuits themselves and the complex logical control circuit additionally added for controlling the bypass switching circuits, so that the conventional multiple power mode power amplifier is considered as regressive considering a trend towards a smaller-sized mobile handset. Also the enlarged size of the entire system is disadvantageous in price competitiveness, particularly since the switches are expensive components.
Therefore, it is a need to provide a more power efficient power amplifier, and in particular, a multiple power mode amplifier that does not use any switches.